A wind turbine is an energy conversion system which converts kinetic wind energy into electrical energy for utility power grids. Specifically, wind incident on blades of the wind turbine generator causes a rotor of the wind turbine generator to rotate. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. Because wind speed fluctuates, the force applied to the WTG blades and hence the rotational speed of the rotor/generator can vary. Power grids however require a constant frequency electrical power to be generated by the wind turbine generator.
A wind power plant is often referred to as a group of wind turbine generators which are commonly connected to an electrical grid through a common connection point, also known as Point of Common Coupling (PCC). Additionally the wind power plant may comprise a power plant controller and/or some sort of reactive power compensation equipment, such as STATCOMs or switch capacitors, or others.
One or more wind turbines of the wind power plant may be requested to shut down, for example due to a fault in the grid, component failure in the wind turbine generator, wind farm or requested by the grid operator. The wind turbine generator reduces its power during shut down, the power produced by the wind turbine decreases to zero at a specified rate. The rate of decrease of power, commonly called the power ramp down rate, is dependent on the power ramp down rate of the generator. How fast the turbine power can decrease during a shut down is normally limited by the maximum ramp down rate of the generator e.g. 0.2 pu/sec.
However, certain grid codes/utilities may require the turbine to ramp down at a rate faster than the maximum ramp down rate of the turbine/generator e.g. 0.4 pu/sec. It is also foreseen that some countries may specify in their grid code requirements a minimum ramp down rate of turbines during shut down.
Some transmission grids may require a rapid decrease in output power from Wind Power Plants (WPP); often such rapid decrease is called a “fast run-back”. This may be triggered by over-frequency excursions or other grid events.
Rapid power reduction may be archived in different ways, various types of turbine may, due to mechanical loads be restricted in its freedom to perform rapid power reductions, performed by fully shutting down the turbine or by blade pitching.
Hence it is desirable to provide a method of operating a wind turbine to provide a power ramp down which causes limited mechanical loads on the wind turbine generator and its components.